Light water moderated neutronic reactor



sept. 17, 1957 R. F. CHRISTY' I'AL y v`v4LIGHT WATER MODERATE!)NEUTRONIC REACTOR Filed Jan. 9. 1946 3 Sheets-Sheet l Avr Sept 17, l957R. F. CHRISTY ErAL 2,806,819

Y LIGHT WATER MODERATED NEUTRONIC REAcToR .Filed Jan. 9, 194s ssheets-sheet 2 Sept. 17, 1957 Y R. F. CHRISTY Er'AL 2,806,819 LIGHTWATER MODERATED NEUTRONIC REACTOR i Filed Jan. 9, 1946 3 Sheets-Sheet 3ilnite Patented Sept. 17, i957' LIGHT WATER MDERATED NEU'IRNEC REACTGRRobert F. Christy, Santa Fe, N. Mex., and Alvin M. Weinberg, Oak Ridge,Tenn., assignors to the United States of America as represented by theUnited States Atomic Energy Commission Application January 9, 1946,Serial No. 640,100

2 Claims. (Cl. 1204-4932) The present invention relates to atomic powerplants, and more particularly to a neutronic reactor composed of uraniumand light water (H2O).

In neutronic reactors, a thermal neutron issionable (herein calledmerely fissionable in accordance with the terminology now commonlyemployed in the art) isotope such as U233, U295 or 94339 or mixturesthereof, is subjected to nuclear fission by absorption of slow neutrons,and a self-sustaining chain reaction is established by the neutronsevolved by the fission. In general such reactors may comprise bodies ofcompositions containing such ssionable material, such as, for example,natural uranium, containing .7% of U235 disposed in a regulargeometrical pattern known as a lattice in a neutron slowing material ormoderator. Graphite, beryllium and heavy water are typical moderatorssuitable for such use. Heat is evolved during the fission reaction andis customarily removed by circulating a coolant in heat exchangerelationship with the uranium. In such reactors, the transuranic element94 (plutonium), is formed as a byproduct of the reaction. Specificdetails of the theory and essential characteristics of such `reactorsare set forth in the copending application of Enrico Fermi and LeoSzilard, Serial No. 568,904, led December 19, 1944, now iatent2,708,656.

In reactors using natural uranium disposed in lattice arrangement ingraphite, beryllium or heavy water, the aggregation of the uranium soreduced loss of neutrons at resonance energies in the U238 content ofthe element, that a chain reaction could readily be attained in areactor of practical size. However, the neutron capture cross section oflight water (H2O) is so high that even with aggregation with optimumvolume ratio of uranium to a light moderator water, the neutronreproduction ratio in a system of infinite size, known as the factor K,would not be suiiiciently over unity to permit a smaller system, thatis, a system of finite and practical size to be built, and still be ableto sustain a chain reaction with a reproduction ratio of unity orbetter. This fact follows, because there could be no external neutronloss in a system of infinite size, whereas in any system of linite sizethere will be an exterior neutron leakage. For a .given lattice andmaterials, the leakage will increase as the device is made smaller. Anysuch leakage causes neutrons to be lost to the chain reaction andreduces the reproduction ratio that can be obtained in the structure tobe built. Thus, a lattice having a K factor of unity cannot support achain reaction in a reactor of nite size.

However, with proper aggregation of natural uranium in an ordinary watermoderator, and with the proper volume ratio of one to the other,resonance losses are reduced to such an extent that the K factor of auraniu1nH2O lattice closely approaches unity. By taking advantage of thereduction of resonance absorption by aggregating the uranium in lightwater, and then increasing the amount of lissionable isotope in theuranium only slightly over that obtained in natural uranium, it has beenfound that a K factor above unity can be obtained and that aself-sustaining chain reaction will occur in a neutronic reactor ofpractical size even though the fissionaL-le isotope content is less than1%, when this enriched uranium is immersed in light water (H2O) as amoderator. Fl-"he term water, as used hereafter in this specification,will be used to refer only to light water.

Uranium may be termed enriched when the lissionable isotope content isabove that occurring in natural uranium. ln this respect, percentageenrichment is referred to as the relative increase in the amount offssionable isotope present in the uranium over that occurring in nature.Natural uranium contains .7% U235. A 20% enriched uranium will contain.84% of U235 or equivalent isotope, 50% enrichment will contain 1.05% ofthe fissionable isotope, and 100% enrichment will bring up theiissionable isotopic content only to 1.4%. Thus, it can be seen thateven with enrichments up to 100%, the uranium will still contain lessthan l1/2% of a tissionable isotope.

it is an object of the present invention to provide a means and methodof creating a self-sustaining chain reaction in a composition of uraniumand water when the iissionable isotope content of the uranium is onlyslightly over that occurring in natural uranium, preferably anenrichment to less than 2% total fissi-enable isotope in the uranium.

Uranium enriched only to the slight degree above referred to cannot,without aggregation in specific geometries and volume ratios, be used inconjunction with water as a moderator to support a chain reaction in areactor of practical size.

it is, therefore, another object of the present invention to provide ameans and method of dispcrsing slightly enriched uranium in water sothat a self-sustaining chain reaction can `be obtained in a neutronicreactor of practical size,

The use of H2O as a moderator in a neutronic reactor is highly desirablefor many reasons. It is cheap, readily available, and can serve whenlattice type compositions are used to support the reaction, both asmoderator, and as coolant to remove the heat of reaction. As the rootmean square distance a neutron has to move between birth as a :fissionneutron to death by absorption as thermal energy is the shortest inwater of all of the above-mentioned moderators, the critical size of thereactor where the reproduction ratio is unity, for a given l( factorwill also be the smallest.

Enrichment .of natural uranium in U235 can be accomplished, for example,by a diffusion barrier isotope separation process wherein naturaluranium hexalluoride (UFG) is passed in gaseous form through a pluralityof porous barriers. The lighter isotope passes through such barriersmore readily than the heavier isotope, and progressive enrichment occursas the heavier isotope is held back. Enrichments of UF6 to 2% U235 canreadily be obtained. The enriched UF6 is then changed to UF4 by ahydrochloric acid treatment, reduced to massive uranium metal byreaction in a bomb with magnesium, and the resultant metal cast,extruded, or otherwise worked and machined into bodies of the size andshape desired.

ln addition, natural uranium can be enriched by adding to naturaluranium other iissionable isotopes such as U233 and 94239. U233 isformed as the result of neutron absorption in thorium. 94239 is formedin any neutronic reactor wherein uranium298 is present, by neutronabsorption therein and subsequent beta decay. Both H293 and 94239 can beobtained in high concentration and purity and added to natural uraniumas desired. As used herein, therefore, the term enriched uranium is tobe understood as meaning U238 combined with more than .7% of any of theiissionable isotopes, or mixtures thereof.

The above objects and advantages of the present invention will be morefully understood from the following detailed'descriptio'n read byreference to the drawings, wherein:

Fig. 1 is a diagrammatic vertical sectional view, partly in elevation,of` an illustrative neutronic reactor system embodying the presentinvention;

Fig. 2 is an enlarged cross-sectional view taken as indicate by the line2.-2 in Fig. 1; g

Fig. 3 is a cross-sectional view through a tube containing a lrod ltypelattice used in the device of Fig. 1; and

Fig. 4 is a vertical cross-sectional view taken as indicated by the line4 4 in Fig. 3.

Referring to Figs. 1 and 2 illustrating a light water moderatedVneutronifc reactor having a lattice Vof only slightly enriched uranium,concrete walls 10 deline a pit 11 of `circular cross-section in thebottom portion thereof,

this bottom portion being lined with a tank 12 having a lower coolingwater inlet 13.

Positioned ,well above theV bottom of tank 12 is an apertured supportinggrid 14 on which is supported a plurality'of hexagonal loosely fittedaluminum tubes 15 grouped in the center of the pit space 11, as shown inFig. 2. A space is thus provided between the group of tubes 15 and thetank 12 to form a reflecting layer 17 of Water when the pit is filledtherewith.

Tubes 15 are set into top and bottom spacer plates 19 and 20,respectively, these plates being provided with apertures 21 to providewater flow through the reflecting layer 17, in lesser amount howeverthan will pass through tubes 15. Tubes 15 are provided on top withhandles 22 by which'they can be removed from the pit by use of a removalrod 24 having a lower hook 25 thereon engageable with a handle 22. Rod24 extends to a platform 2 across the top of the pit.

When removed, tubes 15 are placed in a coffin 28 formed from aradioactivity shielding material such as lead, for example, and disposedon ledge 29 in pit 11 above the reactor structure. Coiin 28 is providedwith a bayonet or pin locked cover 30 operated by coin rod 31 by whichthe col-lin, with its enclosed tube can be removed when desired when thecover is locked in place.

A central tube 15C is vertically movable as by cable 35, drum ,36 andmotor 37, the lat-ter two structures being mounted on platform 26.Control of the reaction is obtained by insertion of more or less of thecentral tube 15C into the reaction zone as defined by the groupedtubes.. The reactivity of the reactor, and the neutron Vdensitydeveloped therein, is. monitored by ionization chamber 38 insertedadjacent the periphery of the reactor reflector.

The uranium lattice use-d inside each tube 15 in a preferred embodimentcomprises spaced uranium rods 40 protected from corrosion by thinaluminum jackets 41;

The mounting system for the rods 40 is shown in Fig. 4. The top andbottom of each tube 15 is closed by supporting end cap plates 42 havingdimples 44 tting coned ends 45 of the uranium rods 40 thus holding therods parallel and in proper lattice spacing in accordance with thevolume ratio `of moderator to uranium desired. End cap plates 42 areperforated with apertures 46 between Y the dimples 44 so that a clearwater ow past all the rod surfaces is obtained when water is forcedupwardly through the tubes 15. The number of tubes 15 used, and thedimensions of Vthe reaction zone formed by the grouped rods 40will'depend on the degree of enrichment, as given in later tables. Thegrouped tubes 15 with rods 40 therein form a substantially continuouslattice throughout the group.

Above the Vreaction zone, the pit 11 may hold up to 50 ft. of water, ascontrolled by the depth -of the pit, the position of an upper wateroutlet 47, and internal pressure as determinedby air inlet 49 into apressure head Y 4 50 sealing the top of the pit 11. This arrangementpermits the reactor to be operated under pressure as desired, to preventboiling of the moderator around the rods at high power. VThe depth ofwater over the reactor acts as yan upper radiation shield.

One of the uses for a reactor of the presently described type,particularly in the smaller sizes, is as a source of neutrons of highdensity. To this end, a thermal neutron column 55 of graphite bricks,for example, is extended laterally from tank 12. All neutrons leavingthis column will be reduced to thermal energy by passage through thegraphite and the external surface thereof can be used as a source ofthermal neutrons for nuclear physics research.

On another side of the reactor, a thimble 56V extends laterally throughthe water reflector 17 almost to the lattice. Fast neutrons can enterthis thimble, and pass outwardly along the thimble and through theexterior opening thereof to provide fast neutrons for nuclear research.Furthermore, materials to be subjected to high density neutronirradiation can 'be placed inside thimble 56 and irradiated close to thelattice.

In operation, the reaction zone is provided with suliicient rods 40 inproper spacing so that a neutron -reproduction ratio of unity isobtained with the central control tube 15C partly withdrawn from thereaction zone. Then byY insertion `of the control tube further into thereactor a reproduction ratio of greater than unity can be obtained. Theneutron density then rises exponentially. When the desired power hasbeen reached, the control tube 15C is withdrawn until the reprodu-ctionratio is again Vunity. T he attained power is thus stabilized. In themeantime, water circulation is maintained through the reactor and thereflecting zone by vcirculation between inlet 13 and outlet47 and theheat of reaction is carried away. Oncethrough water is preferable, aswhen fresh water is used, no great amount of radioactivity is formedtherein and no fission products enter the coolant stream due to thealuminum jackets 41 on the rods 40. However, pumps re-circulating thewater` can be used if desired. The amount ofy water used will, ofcourse, depend on the poweroutput at which the reactor is to operate, aswill the pressure. At powers of 10,000 kw. and above, a pressure of 10latmospheres is preferred to be maintained in the reaction zone.

Having described the physical structure of aV device embodying thepresent invention, the nuclear physics thereof will next'be discussed.

A neutronic reactor, wherein the reactive composition is naturaluranium, will support a chain reaction when the moderatoris D20 .but notwhen H2O is used. This is `because the Yratio of neutron capturecross-section to neutron scattering cross-section of H2O is much higherthan D20, the best presently known values being as follows:

Light water (H2O) V .00478 Heavy water (D20) .00017 Because of thisfact, if natural uranium is disposed inv H2Oias a slurry, for example,of uranium oxide, the K aggregation alone, therefore, raises K fromabout .85`

lin-a slurry to just short of unity when metal rods or spheres are used.V To obtain a K of unity with a slurry, it Iwould Ihave tobe enrichedabout 40% in its fssionable isotope content; However, even then noreactor of practical size could be constructed, as K should be 'at least1.03` to permit external leakage enough for reduction to Sphericalreactors (without reflector) Reactor Enrich- Abun- Uranium Radius, Kment, dance, Mass,

cm. percent percent tons 7G 1. 10 25 89 11. 5 G2 l. 16 40 1.00 6.1235 1. 50 180 2.00 1.07

Volume ratio H2O-U=approx1mately 2.

It will ybe .seen 4from the above that only a 4% enrichment of ltheaggregated uranium will provide a composition having a K of unitywhereas a 40% enrichment of the uranium, when Iin a :slurry form, isrequired to obtain the same K factor.

However, in many cases, a cylindrical reactor lendsV itself .better forpractical construction, and the following values are given forcylindrical 4reactors with the height (H) equal to diameter `(withoutreector):

Radius, Enrich- Abun- Mass U cm. H, cm. K ment, dance, (tons) percentpercent m m 1.00 4 73 m When, `as in the present instance, a waterreector at least 30 cm. thick is positioned around all sides of thereact-ive composition, the reactors can be reduced in size. T-he table.below gives values for reactors having a water reflector formed as -acontinuation outwardly of the water moderator:

Radius, Enrieh- Abun- Mass U cm. H, cm K ment, dance, (tons) percentpercent 62 124 l. 10 25 89 9. 20 49 98 l. 16 40 1.00 4. 47 24 48 l. 50180 2.00 520 For a graphite reector positioned outside of a tankcontaining the lattice, the above dimensions can be again reduced byabout Neutronic reactors of the type described are fully operative atlow powers where little heat is developed -in the uranium when the rodsare used without coatings. Corrosion in such cases can be reduced byusing the water at low temperatures, and by polishing or brightannealing the rods. However, for higher power outputs sealed aluminumjackets applied or bonded to the rods are preferred with the latticesectionalized in aluminum tubes. In this case, about .02% K is lost byneutron absorption in the aluminum. This absorption can be compensatedfor either by raising the enrichment slightly or .by enlarging thereactor to reduce the exterior loss by an amount corresponding to thealuminum loss.

When a substantial power output is to be taken from a reactor of thetype described herein, such as, for example, 10,000 kw. or more, areactor from 60 to 100 cm. in radius is preferred in order that powermay be prop erly dissipated.

It can be seen from the above figures that a reactor utilizing uraniummetal having only a 10% enrichment is of practical size, and in factcontains no more uranium than some natural uranium-graphite reactors andconsiderably less than most high power reactors of that type. 1naddition, when enrichment bringing the issionable isotope content up to2% is used, only .520 ton of uranium is required when a reector is used.At a 2% abundance of the iissionable isotope, this amount of uraniumwill contain only 6.71 kg. of the fissionable isotope.

The figures given above for rod and sphere radii and for volume ratioare optimum values. Departure can be made from these values withoutgreatly altering the amount of iissionable isotope, and it has beenfound that with lumping radii from between .5 cm.. to 1 cm. can be used,with volume ratios of from 1.7 to 2.5 without the required enrichmentrising over about 40% (1.00% abundance) for a reactor well withinpractical size.

It is also to be clearly noted from the above, that the invention asherein described and claimed permits a neutronic reactor to be built inpractical sizes with a light water moderator when the uranium isenriched by a fissionable isotope to an abundance of over .78% and notover 2% in U235, and that-by combining aggregation with enrichment ofthe uranium such reactors can be built of uranium and light water whichotherwise cannot support a chain reaction without much higher enrichmentand use of much larger quantities of the reactive metals.

What is claimed is:

1. A neutronic reactor comprising a plurality of parallel spaced U235enriched uranium rods` having between 0.78% and 2.00% abundance of U235,the rods being 1.8 cm. in diameter, the total mass of uranium rods beingbetween 59.6 tons and 0.520 ton, a light water moderator, the volumeratio of the moderator to uranium rods being two, the uranium rods beingsubmerged in the moderator, the rods and the moderator being disposed inthe shape of a cylinder having height equal to diameter wherein thediameter is between 242 cm. and 48 cm., and a light water reector atleast 30 cm. thick disposed to surround the cylinder on all sides.

2. A neutronic reactor comprising a plurality of parallel spaced U235enriched natural uranium rods having 0.89% abundance of U235, the rodsbeing 1.8 cm. in diameter, the total mass of the uranium rods being 9.20tons, a light water moderator, the volume ratio of the moderator touranium rods being two, the uranium rods being submerged in themoderator, the rods and moderator being disposed in the shape of acylinder having height equal to diameter wherein the diameter equals 124cm., and a light water reector at least 30 cm. thick disposed tosurround the cylinder on all sides.

References Cited in the file of this patent FOREIGN PATENTS 114,150Australia May 2, 1940 861,390 France Oct. 28, 1940 233,011 SwitzerlandOct. 2, 1944 OTHER REFERENCES Roberts et al.: Journal of AppliedPhysics, article entitled Uranium and Atomic Power, vol. 10, 1939, pp.612 to 614.

Chemical Abstracts, 34, 7734 (1940). Abstract of publication byZeldovich et al., in I. EXptl. Theoret. Phys. (U. S. S. R.) 10, 29-36(1940).

A General Account of the Development of Methods of Using Atomic Energyfor Military Purposes, by H. D. Smyth, pub. August 1945, pp. 2l, 22, 23,24, 25, 26, 27.

Smythe: A General Account of the Development of Methods of Using AtomicEnergy for Military Purposes, August 1945, p. 75.

Goodman: The Science and Eng. of Nuclear Power, vol. I, p. 275,Addison-Wesley (1947).

Kelly et al.: Phy. Rev. 73, 1135-9 (1948).

1. A NEUTRONIC REACTOR COMPRISING A PLURALITY OF PARALLEL SPACED U235ENRICHED URANIUM RODS HAVING BETWEEN 0.78% AND 2.00% ABUDANCE OF U235,THE RODS BEING 1.3 CM. IN DIAMETER, THE TOTAL MASS OF URANIUM RODS BEINGBETWEEN 59,6 TONS AND 0.520 TON. A LIGHT WATER MODERATOR THE VOLUMERATIO OF THE MODERATOR TO URANIUM RODS BEING TWO, THE URANIUM RODS BEINGSUBMERGED IN THE MODERATOR, THE RODS AND THE MODERATOR BEING DISPOSED INTHE SHAPE OF A CYLINDER HAVING HEIGHT EQUAL TO DIAMETER WHEREIN THE